We examined the role of (1) priming with live recombinant vaccinia virus vs. subunit antigens, (2) surface antigen gp120 vs. full-length gp160, and (3) the combination of envelope and core antigens in eliciting protective immunity. Twenty-four pigtailed macaques were immunized according to the regimen in the table and were challenged intravenously with 20-200 MID of SHIV HXBc2 four weeks after the last boost. Group Prime/Boost DOC (-gp120 EIA titer DOC SHIV-neut Inf'd/Total A rgp120/rgp120 2,369 (1,377-8,166) <20 (<20-39) 3/3 B rgp160/rgp160 894 (236-47,863) 27 (<20-103) 2/4 C Vac-gp160/rgp120 20,000 (12,794-53,703) 220 (154-382) 0/3 D Vac-gp160/rgp160 22,156 (11,776-43,351) 273 (165-446) 0/3 E Vac-gag pol env/particles 1,385 (638-4,315) 25 (<20-33) 2/5 F Vac-NY/mock <50 <20 4/4 Complete protection was observed in all 6 macaques in groups C and D, as determined by negative DNA and RNA PCR, virus isolation and anamnestic response. Protection was correlated with high anti-gp120 antibody titers and the presence of neutralizing antibodies. Three macaques in group E, which failed to generate significant levels of neutralizing antibodies, were nevertheless protected, indicating a possible role of cross-reactive immune responses against core antigens in protection. As observed in our first study, one of the four infected control macaques developed CD4 decline and was euthanatized due to AIDS, indicating the pathogenic potential of the challenge virus in the pigtailed macaques. Ten animals from groups B, C, D, and E that were completely or partially protected against SHIV HXBc2 challenge are being held for rechallenge. FUNDING NIH grant RR00166.